This invention relates to an improved hydrocarbon hydrocracking process. More particularly, it relates to upgrading bottoms of a resulting hydrocrackate product by selective extraction of residual nitrogen-containing and/or polycyclic hydrocarbon impurities from the bottoms.
Nitrogen-containing and polycyclic hydrocarbons are known to adversely affect typical hydrocarbon hydrocracking processes. The former impurities are selectively adsorbed by acid sites (see, for example, U.S. Pat. No. 3,288,703) of the catalyst, thereby inhibiting the desired cracking activity of the catalyst. The latter impurities may also be selectively adsorbed with a like catalyst-inhibiting effect. They are also notorious in their role as precursors of coke formed during hydrocracking. Coke buildup on the catalyst surfaces also interferes with desired catalytic effects and process efficiency. The foregoing effects are especially notable where the bottoms of the hydrocrackate are recycled to the process or used in whole or part as feed to a subsequent hydrocracking process stage. In the recycle mode of operation, these residual impurities tend to persist and build up concentration-wise in the process streams because of their resistance to cracking. Consequently, the relative amount of crackable components of the composite feed to the process declines and the process efficiency also declines. A reject stream is usually employed as a countermeasure. However, this stream normally contains a major portion of crackable feed components which are lost to the process.
An object of this invention is to provide an efficient and economic method for removing residual nitrogen-containing and/or polycyclic hydrocarbon impurities from a hydrocrackate product, especially the bottoms fraction, which is typically subjected to further hydrocracking conditions.
Other objects will become apparent from the following examples and description of the invention.